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Joint Astrophysics ColloquiumJoint Astrophysics SeminarProperties of the Close Binary and Circumbinary Torus of the Red RectangleAlexander MenshchikovMax Planck Institute for Radio AstronomyNew diffraction-limited speckle images of the Red Rectangle in the wavelength range 2.1-3.3 mic with angular resolutions of 44-68 mas and our previous speckle images at 0.7-2.2 mic revealed well-resolved bright bipolar outflow lobes and long X-shaped spikes originating deep inside the outflow cavities. This set of high-resolution images stimulated us to reanalyze all infrared observations of the Red Rectangle using our two-dimensional radiative transfer code. The high-resolution images imply a geometrically and optically thick torus-like density distribution with bipolar conical cavities and are inconsistent with the flat disk geometry frequently used to visualize bipolar nebulae. The new detailed modeling, together with estimates of the interstellar extinction in the direction of the Red Rectangle enabled us to more accurately determine one of the key parameters, the distance D = 710 pc, which is twice as far as the commonly used estimate of 330 pc. The central binary is surrounded by a compact, massive (M = 1.2 Msun), very dense dusty torus with hydrogen densities reaching n_H = 2.5 x 1012 cm-3 (dust-to-gas mass ratio ρd/ρ ~ 0.01). The model implies that most of the dust mass in the dense torus is in very large particles and, on scales of more than an arcsecond, the polar outflow regions are denser than the surrounding medium. The bright component of the spectroscopic binary HD 44179 is a post-AGB star with mass M* = 0.57 Msun, luminosity L* = 6000 Lsun, and effective temperature T* = 7750 K. Based on the orbital elements of the binary, we identify its invisible component with a helium white dwarf with Mwd ~ 0.35 Msun, Lwd ~ 100 Lsun, and Twd ~ 6 x 104 K. The hot white dwarf ionizes the low-density bipolar outflow cavities inside the dense torus, producing a small HII region observed at radio wavelengths. We propose an evolutionary scenario for the formation of the Red Rectangle nebula, in which the binary initially had 2.3 and 1.9 Msun components at a separation of 130 Rsun. The nebula was formed in the ejection of a common envelope after Roche lobe overflow by the present post-AGB star.
Thursday, January 16th 2003, 12:30
D-423, Département de Physique, Université de Montréal |